Anodic Stability of New Electrolyte Containing Cyano-Substituted Benzimidazole Derivative Lithium Salt: New Insights By in-Situ Drifts Analysis

Through the increasing demand of lithium ion batteries (LIBs) for high potential and high temperature operating conditions, LIBs with improved electrochemical performance and safety become an important research goal. This especially holds true for LIBs used in high powered electric vehicles (EVs), hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs) and smart grids [1]. However, the development of electrolytes and electrode materials in LIBs is facing some technical bottlenecks at this stage. For instance, it is quite difficult to find stable electrolyte and electrode materials that can work at high potentials (> 4.5 V vs. Li⁺/Li), because of the insufficient thermal and electrochemical stability of conventional electrolytes and cathode materials [1-3]. The conventional electrolytes (e.g.,LiPF₆ with organic carbonate solvents) and cathodes (e.g., LiCoO₂) have limited electrochemical window (<4.3 V) [1-5]. To circumvent this drawback, Chern et al. [6] recently reported a new lithium salt, a cyano-substituted benzimidazole derivative Li-salt (Li[5-CNTFBI(BF₃)₂]), for high potential and high temperature applications of LIBs. This study reports the use of in-situ DRIFTS (diffuse reflectance infrared Fourier-transformed spectroscopy) technique [7] for analysis of SEI (solid electrolyte interface) formation when using the new electrolyte. The new Li[5-CNTFBI(BF₃)₂]electrolyte demonstrated improved anodic stability comparing to conventional LiPF₆electrolyte.

From the CV curves (Fig.1), the redox peaks centered at 4.07 and 4.34 V for LiPF₆ and Li[5-CNTFBI(BF₃)₂], respectively, over LiCoO₂. This indicates that the new electrolyte improved anodic stability comparing with conventional one. Fig.2 illustrates the in-situ DRIFTS spectra at OCV of LiPF₆ /EC+DEC and Li[5-CNTFBI(BF₃)₂]/EC+DEC over LiCoO₂ working electrode. The two spectra exhibit absorbance bands, which can be assigned as mainly from contribution of both the thin layer of electrolyte (Li-salts and organic solvents) and surface adsorbed species. The difference spectra during stepwise potential scan were also analyzed to evaluate the anodic stability of the electrolyte as well as the SEI formation as shown in Fig.3. On the commercial LiCoO₂ working electrode, the SEI species were observed starting from 4.0 [8] and 4.4 V, respectively, for LiPF₆ and Li[5-CNTFBI(BF₃)₂] which is in good agreement with CV result. SEI species observed along with absorbance band broadening attributable to intermolecular hydrogen-bonding of–CN and –BF groups above the mentioned onset potential were somewhat different for the two electrolyte systems.  The identified species and their possible formation mechanism will be discussed on high capacity Li-rich (LLNMO) and LiCoO₂ cathode materials.

Acknowledgement

We are grateful to Ministry of Economic Affairs of Taiwan, ROC (103-EC-17-A-08-S1-183)for the financial support.

References

[1] S.Tan, Y.J. Ji, Z. R. Zhang, and Y. Yang,Chem Phys Chem, 15 (2014) 1956-1969.

[2] S. S. Zhang, Journal of Power Sources,162(2006) 1379-1394.

[3] K. Xu, Chem. Review, 104 (2004) 4303-4418.

[4] J. B. Googenough, Y. Kim, Chem. Mater.22 (2010) 587-603.

[5] K. Xu, Chem. Review, 114 (2014) 11503-11618.

[6] Y. - T. Chern, J.- L. Jeng,  S. – Y. Chen,  A. – S. Wei, 

     B. – J. Hwang, Patent, US 2014/0178771 A1,

[7] A. M.  Haregewoin, T. - D. Shie, S. D. Lin, B. - J. Hwang, F. - M. Wang, ECS Trans.53 (36)(2013) 23-32.

[8] M. A. Teshager, S. D. Lin, B. J.Hwang, F.-M.Wang, S. Hy, A. M. Haregewoin, Electrochemica Acta (submitted).